Actually, there may be a (very imperfect and cumbersome) way to do it.
You could add your custom tags to the ‘genre’ field, which is a multi-value tag (at least in most file formats); this won’t always be easy or feasible, depending on how many custom tags you set up and how they’re meant to interact with each other, but, in a simple case, you could add, say, ‘Music my partner hates’ to the genre field rather than to a custom field.
You’d then need to set Roon so that it reads your custom tags:
https://kb.roonlabs.com/Import_Settings
At this point you should be able to filter based on these ‘custom genres’.
The knowledge base says Roon can use both its genres and those in my tags, but I’m not sure if the interaction would produce a messy result. I’m only on a trial licence for now and I’m not sure I’ll have the time to test this thoroughly in the next few days. However, I’m hoping this can be useful food for thought!
Edit: PS Unfortunately, you cannot use ‘focus’ on an Android phone, but you can use ‘focus’ on a desktop, set a bookmark, and use the bookmark from the phone. I posted a feature request: Better scrolling for Android phones: add Focus and scroll bar/first letters